Killer Michael Chudley, 63, who shot dad-of-three James Ward, 58, in his office at MGW Law, in Devizes, in July 2012 has been found dead in his prison cell.
Chudley was initially accused of attempted murder, but his charge was upgraded when Mr Ward died three weeks later.
Chudley, who would have been 64 on Friday, claimed diminished responsibility but was convicted by a jury of murder at Salisbury Crown Court last June and jailed for life.
He was serving at least 28 years' imprisonment and last week he failed in a Court of Appeal bid to clear his name.
A Prison Service spokesperson said today: "HMP Winchester prisoner Michael Chudley was found unresponsive in his cell shortly before 5.50am on Tuesday.
"Paramedics attended and attempted CPR but he was pronounced dead at 6.36am.
"As with all deaths in custody, the Prisons and Probation Ombudsman will conduct an investigation."
Chudley, a bankrupt property developer, blamed Mr Ward for the break-up of his relationship and the repossession of his £1 million home - Kingfisher House in Rowde, near Devizes.
The crown court heard he walked into the solicitor's office and shot him in the head once at close range with a sawn-off shotgun as he spoke to a client on the telephone.
The force of the blast blew out a window of his first-floor town centre office.
Mr Ward, who was known as Jim, suffered catastrophic head injuries and died almost three weeks later in hospital.
Mr Ward was married with three children and lived in Bratton. His wife, Nicky Morris, also a solicitor, was unavailable for comment about Chudley’s death.
Chudley, who had changed his name from Mike Russell, had lost his home and his business after becoming embroiled in a long-running legal dispute over building work.
The man he unsuccessfully sued, Christopher Sear, had instructed Mr Ward, and Chudley was facing the prospect of paying legal costs of more than £250,000.
Prosecutors said Chudley, who by this time had either been renting rooms in local pubs or sleeping in his car, acted out of revenge, anger and resentment.
He had denied murder but had admitted manslaughter on the grounds of diminished responsibility, claiming he was suffering from an abnormality of mental function at the time.
But the jury took little more than two hours to find him guilty of murder.
The judge, Mr Justice Bean, sentenced Chudley to life imprisonment with a minimum term of 28 years.
He said the murder of Mr Ward was a “chilling, calculated execution” and described Chudley as wallowing in self pity.
Mr Justice Bean said Chudley would be 90 before he could be considered for release after serving his 28-year-term.
He said: “It may be that you will die in prison. But that is a possibility for which you have only yourself to blame.”